Due to their low power consumption and small size, flat-panel displays have become increasingly popular for such applications as lap-top computers and pocket television receivers. See "Flat-Panel Displays," Scientific American, March 1993, p. 90. Transmissive displays such as liquid-crystal (LCD) displays have been especially well received due to their extremely low power consumption and potential for high resolution, including full-color applications. Whether of the twisted nematic or active-matrix variety, however, such LCD displays require backlighting, as they are non-emissive.
Regardless of whether the display is back-lit directly or indirectly, some form of light diffuser is generally required. Direct illumination sources include an array of tubular lamps or a single serpentine-type of lamp. Indirect sources often take the form of an edge-lit substrate composed of a material having light-piping characteristics, with light introduced along an edge being emitted across the surface after undergoing reflections internal to the light pipe. The diffuser is then usually supported against this light-emitting surface.
Conventional diffusers are ordinarily formed by a process which produces a roughened surface or which results in embedded scattering sites. Commonly used materials include flashed opal, sand-blasted glass and various flexible films exhibiting light-diffusing properties.
In general, conventional diffusers suffer from low efficiency. This is largely due to the fact that the axis associated with the diffused light is usually substantially the same as the ray of incident light, which has a low probability of already being normal to the surface of the diffuser--that is, in the direction of the display panel. Furthermore, even if the incident light and the scattered light are in the general direction of the display panel, conventional diffusers are prone to backscatter which further limits their overall efficiency.
Generally speaking, efforts to improve the uniformity of a conventional diffuser further degrade their efficiency. For example, by stacking conventional diffusers or using multiple diffusers which scatter through a variety of angles, uniformity will be improved, but backscatter and transmission loss will be experienced at each diffusing stage. Multiple panels also tend to increase manufacturing costs, or the depth of the backlighting structure, or both.
It has been proposed to use holographic-type diffusers in conjunction with flat-panel displays such as LCDs, but for the purpose of providing a uniform readout when viewed at an angle oblique to the display surface. As such, these diffusers are placed between the display and the observer, and not between the source of illumination and the back of the display panel. In U.S. Pat. No. 5,037,166, for example, a holographic optical system is used to redirect a display image only to an area at which the viewer may observe the image so as to prevent unwanted reflections and glare as when the instrument panel is within an aircraft cockpit. In U.S. Pat. No. 5,046,793, a holographic diffuser is used to provide chromatic correction, or color balance, in addition to redirecting display information to one or more viewing locations oblique to the display surface.
As such, an unfulfilled need remains for a diffuser, including a holographically prepared diffuser, for use with flat-panel displays such as LCDs. The proposed device should improve the efficiency and uniformity of illumination while allowing the backlight assembly to remain sufficiently thin for lap-top computer and other portable applications.